Active Noise Control (ANC) systems attenuate undesired noise using feedforward and feedback structures to adaptively remove undesired noise within a listening environment, such as within a vehicle cabin. ANC systems generally cancel or reduce unwanted noise by generating cancellation sound waves to destructively interfere with the unwanted audible noise. Destructive interference results when noise and “anti-noise,” which is largely identical in magnitude but opposite in phase to the noise, combine to reduce the sound pressure level (SPL) at a location. In a vehicle cabin listening environment, potential sources of undesired noise come from the engine, the interaction between the vehicle's tires and a road surface on which the vehicle is traveling, and/or sound radiated by the vibration of other parts of the vehicle. Therefore, unwanted noise varies with the speed, road conditions, and operating states of the vehicle.
A Road Noise Cancellation (RNC) system is a specific ANC system implemented on a vehicle in order to minimize undesirable road noise inside the vehicle cabin. RNC systems use vibration sensors to sense road induced vibrations generated from the tire and road interface that leads to unwanted audible road noise. This unwanted road noise inside the cabin is then cancelled, or reduced in level, by using speakers to generate sound waves that are ideally opposite in phase and identical in magnitude to the noise to be reduced at the typical location of one or more listeners' ears. Cancelling such road noise results in a more pleasurable ride for vehicle passengers, and it enables vehicle manufacturers to use lightweight materials, thereby decreasing energy consumption and reducing emissions.
RNC systems are typically Least Mean Square (LMS) adaptive feed-forward systems that continuously adapt W-filters based on both acceleration inputs from the vibration sensors located in various positions around a vehicle's suspension system and on signals of error microphones located in various positions inside the vehicle's cabin. RNC systems in vehicles are susceptible to the noise floor from the vibration sensors or microphones undesirably adding to the total noise within the passenger cabin. The noise floor is the level of background noise in a signal, or the level of noise introduced by the system, below which the signal that's being captured cannot be isolated from the noise. For instance, the noise floor for a vibration sensor, such as an accelerometer, is the output signal it has when it is not subjected to any input vibration. An ideal accelerometer would have an output signal with zero amplitude when subjected to zero road input vibration. A real accelerometer output signal in this case would not be zero, but would have a very small amplitude. Because most RNC systems are feed-forward systems, non-zero noise floor signals from the vibration sensors and/or microphones are amplified and radiated by speakers into the passenger cabin as airborne anti-noise. On certain roads at certain speeds (e.g., on a smooth road at low speed), the sensor noise floor may be audible inside a vehicle with a low in-cabin noise floor, much to the annoyance of passengers.